The present invention generally relates to switching power supply circuits, and more particularly to a switching power supply circuit which can operates a secondary side output voltage detection circuit even in response to a low output voltage.
In transmission systems, switching systems and the like, a driving power supply for electronic circuits uses an A.C. converter system in order to avoid undesirable effects of external noise, surge and the like. Hence, a D.C. power supply which is isolated from a main power supply via a transformer is used as the driving power supply for supplying a voltage lower than a voltage supplied by the main power supply. For example, the main power supply supplies a voltage of -48 V. For this reason, a detection signal of an output voltage detection circuit provided on a secondary side of the transformer is used to carry out a control on a primary side via a photocoupler or the like, so as to stabilize an output voltage.
Conventionally, a D.C. voltage of 5 V, 12 V or the like is supplied by the above described driving power supply for the electronic circuits. However, the voltage supplied by the driving power supply has decreased as the integration density of the electronic circuits (that is, integrated circuits or ICs) improved, and there are demands to enable the use of a voltage of 3 V or less.
However, if an output voltage of a switching power supply circuit is low, the output voltage detection circuit provided on the secondary side cannot operate. Hence, in such a case, it is necessary to provide an independent power supply circuit for the output voltage detection circuit so that the switching power supply circuit can operate.
FIGS. 1A and 1B are circuit diagrams for explaining a conceivable primary control type switching power supply circuit. FIG. 1A shows the primary control type switching power supply circuit, and FIG. 1B shows an output voltage detection circuit of the primary control type switching power supply circuit in more detail.
In FIG. 1A, a transformer T is used for A.C. conversion and includes windings N1, N2 and N3. A switching transistor Tr has a base that receives a driving signal from a control circuit 1, and intermittently supplies a current from a -48 V power supply via the primary winding N1, so as to generate a rectangular wave voltage at the secondary winding N2. The control circuit 1 operates in response to a power supply voltage from the -48 V power supply received via a resistor R0 at the time of a start. After the start, the control circuit 1 operates in response to a voltage that is obtained by rectifying a voltage generated at a third winding N3 by a diode D0.
A light receiving side PC2 of a photocoupler receives light from a light emitting side PC1 of the photocoupler of an output voltage detection circuit 2 which is provided on the secondary side. The light receiving side PC2 supplies to the control circuit 1 a control signal which varies depending on a magnitude of an optical signal received from the light emitting side PC1. Hence, the control circuit 1 controls the driving signal supplied to the switching transistor Tr in response to the control signal, so as to maintain an output voltage V.sub.0 constant.
Diodes D1 and D2 rectify the rectangular wave voltage at the secondary winding N2. A choke L1 and a capacitor C0 form a smoothing circuit, and a rectified voltage obtained via the diodes D1 and D2 is passed through this smoothing circuit so as to generate an output voltage V.sub.0 of +5 V.
An output voltage detection circuit 2 includes a variable constant voltage generation circuit IC1, and has a circuit construction shown in FIG. 1B. In FIG. 1B, an operational amplifier OP1 compares a voltage V.sub.i that is obtained by dividing the output voltage V.sub.0 by resistors R1 and R2 with a reference voltage V.sub.ref, and generates an output voltage V.sub.OP depending on an error voltage that is obtained as a result of the comparison. Hence, a voltage V.sub.PC1 of the light emitting side PC1 of the photocoupler-changes, and the magnitude of the optical signal generated from the light emitting side PC1 accordingly changes. Since the light receiving side PC2 of the photocoupler receives this optical signal from the light emitting side PC1, a control is carried out via the Control circuit 1 so that the output voltage V.sub.0 is maintained constant.
In this state, the following relationship stands between the output voltage V.sub.0 and the voltages at various parts of the output voltage detection circuit 2, where V.sub.R3 denotes a voltage drop across a resistor R3 that is inserted in series with the light emitting side PC1 of the photocoupler. EQU V.sub.0 =V.sub.R3 +V.sub.PC1 +V.sub.OP ( 1)
If V.sub.REF =2.5 V, EQU V.sub.OP &gt;2.5 V (2)
Generally, EQU V.sub.PC1 &gt;1.5 V (3)
Accordingly, the output voltage V.sub.0 can be described as follows based on the relationships (1) through (3) above. EQU V.sub.0 &gt;4 V
According to the conceivable primary control type switching power supply circuit shown in FIG. 1, it is impossible to operate the output voltage detection circuit 2 on the secondary side if the output voltage V.sub.0 is set to a low voltage of 3 V, for example.
FIG. 2 is a circuit diagram showing another conceivable primary control type switching power supply circuit. In FIG. 2, those parts which are the same as those corresponding parts in FIGS. 1A and 1B are designated by the same reference numerals, and a description thereof will be omitted.
In FIG. 2, the transformer T includes a fourth winding N4. In addition, a rectifying diode D3, a resistor R5 and a smoothing capacitor C2 are additionally provided.
In FIG. 2, the fourth winding N4 is connected in series to the secondary winding N2. The diode D3 rectifies a voltage which is obtained by adding voltages generated by the secondary and fourth windings N2 and N4. This rectified voltage is supplied to a smoothing circuit which is made up of the resistor R5 and the capacitor C2, so as to generate an output voltage V.sub.1. This output voltage V.sub.1 is applied to the resistor R3, the light emitting side PC1 of the photocoupler, and the variable constant voltage generation circuit IC1.
Due to the provision of the fourth winding N4, the output voltage V.sub.1 can be made higher than the output voltage V.sub.0 by an arbitrary amount. Hence, according to the primary control type switching power supply circuit shown in FIG. 2, the output voltage detection circuit 2 can operate even if the output voltage V.sub.0 is set to a low voltage of 3 V, for example.
Therefore, in the case of the conceivable primary control type switching power supply circuit shown in FIG. 1, there was a problem in that the output voltage detection circuit 2 on the secondary side cannot operate if the output voltage V.sub.0 is a low voltage of 3 V, for example.
On the other hand, it is conceivable to provide the fourth winding N4 as shown in FIG. 2 so as to generate a higher driving power supply voltage, so that the output voltage detection circuit 2 can operate. However, in this case, there was a problem in that it is difficult to reduce the size and thickness of the transformer T because of the increased number of windings. In other words, it was difficult to satisfy limitations such as a number of terminals, areas of windows for the windings in an iron core and the like which form the restricting conditions related to reducing the size and thickness of the transformer T.